How well do you really know your penis? Find out the five penis facts that every man should know.

You've grown up together and shared many experiences, but how well do you really know your penis? Here are five penis facts you probably didn't know.

You can break your penis

If the penis is violently twisted when erect, it can break. There are no bones in the penis, but the tubes that fill with blood during an erection can burst. Blood pours out of them inside the penis and causes a very painful swelling. Reported cases of penile fracture are rare, but it's thought that some men are too embarrassed to report it to their doctor. Damage during sex, where a partner is on top, is responsible for about one-third of all cases. The breakage usually occurs when a man's penis slips out of his partner and is violently bent.

Men have several night-time erections

On average, a healthy man has three to five erections during a full night's sleep, with each erection lasting 25-35 minutes. It's common for men to wake up with an erection, informally called a "morning glory". This is in fact the last in the series of night-time erections. The cause of night-time erections isn't fully understood. However, studies suggest they are closely associated with the phase of sleep known as REM (rapid eye movement) sleep. This is when dreaming is most common. Whatever their cause, most doctors agree that night-time erections are a sign that everything is in working order.

Penis length is not linked to foot size

The idea that the size of your penis is in proportion to your shoe size is a myth, according to a study published in the British Journal of Urology International. Researchers at University College London measured the penises of 104 men, including teenagers and pensioners. The average penis length in this group was 13cm (5.1 inches) when soft and gently stretched, and the average British shoe size was nine (43 European size). However, researchers found no link between shoe size and penis length.

Small penises make big erections

Shorter penises increase more in length than longer ones when they become erect. Research based on the penis measurements of 2,770 men found that shorter penises increased by 86% when erect, nearly twice that of longer penises (47%). In the 1988 study published in the Journal of Sex Research, researchers also found that the difference in length between a short penis and a longer one was a lot less obvious when erect than when flaccid. For example, the flaccid penises varied in length by 3.1cm (1.2 inches), whereas the average erect lengths differed by only 1.7cm (0.67 inches).

The penis is not a muscle

Contrary to popular belief, the so-called love muscle doesn't contain any muscles. That's why you can't move it very much when it's erect. The penis is a kind of sponge that fills with blood when a man is sexually excited. Blood builds up inside two cylinder-shaped chambers, causing the penis to swell and stiffen. The swelling blocks off the veins that normally take blood away from the penis. As an erection disappears, the arteries in the two chambers narrow again, allowing blood to drain away from the penis.